A new find of boninite dyke from the Palaeoproterozoic Dongargarh Super group: Inference for a fossil subduction zone in the Archaean of the Bastar craton, Central India

Abstract

The Dongargarh Supergroup (DSG), a bimodal Large igneous province (LIP), is one of the Palaeoproterozoic greenschist facies-metamorphosed volcano-sedimentary belts in the Bastar craton of the Central Indian shield. Two contrasting models are in vogue for the generation of the mafic volcanics from the DSG - a continental rifting model and an arc related model. In this paper, we report the occurrence of a boninite dyke from the Bijli rhyolite Formation, which is the lower volcanic horizon in the Nandgaon Group of the DSG. The boninite dyke is characterised by high magnesium (MgO : 18.32-18.80 wt.%), primitive Mgnumber (Mg# > 80), abundance of silica (SiO 2 : 51.63-51.95 wt.%), high Ni (~369 ppm), Cr (~2703 ppm), extremely low titania (TiO 2 : 0.04 wt.%), enrichment of LREE over MREE and HFSE and pronounced negative anomalies in Nb, Ti and Zr on primitive mantle normalized multi-element plots. The Dongargarh boninite dyke is inferred to have been derived from a primary magma and shares geochemical characteristics of modern- as well as Archaean-boninites. It comes under the high-Ca boninite category and displays distinct geochemical traits compared to the so far reported boninites from the Bastar craton. Its petrogenesis necessitates a two stage-model involving a refractory mantle as well as fluids derived from subducted sediments. Crustal assimilation (contamination) or a direct plume-derived melt cannot account for its observed geochemical characters. Even though we cannot constrain the generation of the mafic volcanics of DSG vis-a-vis rifting vs convergence with the available data, the occurrence and geochemistry of the boninite dyke indeed demonstrates that this domain represents a fossil subduction zone.